Fresh ripe fruits are considered by consumers as the ultimate in aroma, flavour, texture and colour. The enjoyable culinary experience of fresh fruits can be attributed to the pleasant balance of sweetness and sourness, the desirable textural firmness and crunchiness, and the richness in flavour notes. From the standpoint of ease of serving and eating, whole fruits are frequently peeled, cored, destoned, deseeded, segmented, sliced and diced in the home and in food service establishments. Packaged, pre-cut fresh fruits are appealing to consumers and food service operators with the benefits of ready-to-use convenience, freshness, fullness of natural flavour, storability and no waste.
Packaged, fresh cut apple pieces could be a year-round item in the marketplace since whole fresh apples can be stored for up to ten months under controlled atmosphere (CA) conditions, and since apple imports are available at other times. For the successful retail and food service marketing of packaged fresh cut apple pieces, the high quality attributes of the cut apple pieces must be retained during storage in containers for periods up to six weeks at refrigerated temperatures. Further, value-added features such as adjunctive flavours and nutrient supplementation would be consumer-appealing for the cut apple items. Such cut apple pieces could be used as a snack, as an ingredient for mixed fruit salads, cakes, tarts, pies and dessert topping or for juice preparation.
The ascorbic acid content of common apple cultivars at harvest is between 2 and 11 mg per 100 grams, which is considered to be one serving of slices. Ascorbic acid supplements would be nutritionally beneficial to bring the level up to at least 60 mg per serving of apple pieces, 60 mg being the recommended daily intake for vitamin C in Canada and the U.S.A.
Further, since consumer appeal is towards food products with no or few food additives, the ascorbic acid (erythorbic acid as an alternative) and the adjunctive flavour ingredients (preferably natural) are to be the only food additives for inclusion in cut apple pieces under the conditions of this invention.
Edible apple tissue consists essentially of interconnected parenchyma cells. The pectic substances (protopectin) in the middle lamella between the cells provide cell--cell adhesion, the adhesion strength being related to tissue firmness and crispness. Turgid parenchyma cells with high osmotic pressures contribute to the textural characteristics of crunchiness of apple tissue. In the mature apple parenchyma tissue, about 20 to 25% of the total volume is made up of intercellular gas spaces. When apple slices are immersed in an anti-browning solution containing ascorbic acid or sulfite, the fluid migrates into the tissue through these intercellular spaces. In the vacuum infiltration technique for the rapid penetration of anti-browning solutions into apple slices, the intercellular gas withdrawn from the apple tissue is displaced by the solution. The shortcoming of such a technique is the creation of undesirable water-logged apple slices.
During the coring, peeling and slicing of whole apples, the mechanical shearing of the parenchyma tissue brings about the rupture of cells, bruising of tissue and decompartmentalization of cellular components (Powrie and Skura, 1991, in Modified Atmosphere Packaging of Food, Ellis Horwood). Such tissue alterations lead to quality deteriorative changes such as:
1. increase in respiration rate; PA1 2. acceleration in ripening and senescence; PA1 3. reduction is tissue firmness and crispness; PA1 4. enhanced enzymatic browning; and PA1 5. increased susceptibility to microbial invasion and deterioration. PA1 (a) Immersion of cut apple pieces in low pH (2.2 to 2.7) solutions with high concentration levels (5 to 15%) of only ascorbic acid, as the acidulant and the reducing agent, for a short period of time (about 30 seconds to 3 minutes) for the purpose of: (1) diffusing ascorbic acid into apple tissue to levels of 200 to 600 mg per 100 grams of cut apple pieces for vitamin C supplementation, inhibition of enzymatic browning and decrease of the oxidation-reduction potential to impede the growth of spoilage organisms such as Acetobacter, yeast and mold; (2) dislodging spoilage organisms from the surfaces of cut apple pieces; (3) stressing the spoilage organisms on the cut surfaces by the high levels of hydrogen ions (low pH) from the ascorbic acid; and (4) removal of decompartmentalized cellular juice on the cut surfaces of the apple pieces. PA1 (b) Prompt removal of portion-excess solution of ascorbic acid by vibration of cut apple pieces and/or by high-velocity gas impingement on the surfaces of cut apple pieces to bring about a range of specific residual surface-liquid levels of 0.5 and 4.0 grams per 100 grams of apple pieces for the purpose of: (1) prevention of the water-logging of tissue (inward movement of excessive amounts of ascorbic acid solution) and prevention of tissue sugar diffusion out of the tissue; (2) reducing the amount of aqueous microbial habitat on the cut apple piece surfaces to limit the total microbial biomass buildup during the storage period with the consequence of impeding microbial spoilage; (3) preventing the formation of free liquid drip at the bottom of a container. PA1 (c) Application of liquid adjunctive flavour ingredient (flavourant) onto the just-moist surfaces of cut apple pieces by atomization of the liquid by means of a spray nozzle with the droplets having diameters of about 40 to 200 micrometers. The small droplets are to travel into the interpiece spaces for uniform deposition onto the surfaces of the apple pieces. A spray nozzle designed for turbulent flow of the droplets will enhance the uniform deposition which ensures multi-direction infusion of the flavourant into each cut apple piece. PA1 (d) Input of a specified gas mixture into the headspace of a container to ensure the presence of an adequate amount of oxygen for the continuance of aerobic respiration at the beginning of MAP storage and for the gradual in vivo production of respiratory carbon dioxide as a functional gas for impeding the respiration rate, ripening, senescence and microbial growth. For apple cultivars with relatively high respiration rates in the cut form, the addition of carbon dioxide to the input gas mixture may be beneficial for reducing the respiration rate and ripening at the beginning of MAP storage. The present volume of oxygen in the input gas is to be 15 to 30, and the optional carbon dioxide is to be in the mixture at levels between 0 and 15 percent by volume. PA1 (e) Apportionment of headspace volume and product volume, being between 0.3:1 and 2:1, in containers for ensuring an adequate amount of oxygen in the headspace to maintain an aerobic microatmosphere and for creating an optimal equilibrium gas mixture in the headspace within a reasonably short storage period. PA1 (f) Containerization of cut apple pieces in a gas-permeable plastic polymeric package having a specified range of gas permeability values required for adequate influx of air into the headspace and outflow of carbon dioxide to result in an optimum equilibrium gas microatmosphere, which is required to inhibit microbial growth, ripening and senescence, to modulate respiration, and in addition, complement the retardation of the enzymic browning by ascorbic acid. The walls of the containers are to have gas permeabilities of 50 to 300 cu. cm. of O.sub.2 per 100 sq. in. per 24 hrs. at 25.degree. C. at 1 atm., and 200 to 1200 cu. cm. of O.sub.2 per 24 hrs. at 25.degree. C. at 1 atm. PA1 (g) Quick-chilling of the cut apple pieces in the container to temperatures of 0 to 4.degree. C. for 24 hours for the purpose of cold shock inactivation of bacteria, which have contaminated the cut surfaces during the cutting and handling of the apple pieces. Cold shocking can inactivate many types of spoilage bacteria, particularly gram-negative organisms. Further, the quick-chilling of apple pieces will lower the respiration rate to a level at which a slow rate of carbon dioxide production occurs in the mitochondria without causing carbon dioxide stress and damage to intact apple tissue. PA1 1. inhibition of microbial growth; PA1 2. reduction of respiration rate; PA1 3. retardation of ripening and senescence; PA1 4. restriction of tissue discolouration even after the product is exposed to air after opening the container; PA1 5. inhibition of off-flavour development; PA1 6. retention of naturally-occurring apple flavour; PA1 7. absorption of adjunctive flavour components by tissue; PA1 8. retention of original crispness; PA1 9. retardation of drip formation; PA1 10. increase in vitamin C; PA1 11. prevention of the development of carbonated taste (fizziness); and PA1 12. restriction of tissue water-logging. PA1 1. gently pressing the surfaces of a specific number of cut apple pieces onto water-absorbent paper such as hand paper towel or filter paper; PA1 2. measuring the weight of the paper before and after apple piece pressing (weight of residual surface-liquid); PA1 3. measuring the total surface area of the cut apple pieces; and PA1 4. calculating the weight of residual surface-liquid per 100 square centimeters of surface area.
When whole apples with respiration rates in the vicinity of 1 mL CO.sub.2 /kg/hr. at 0.degree. C. are cut into slices, the rate of respiration increases considerably and is cultivar-dependent. Kim, Smith and Lee (J. Food Sci. 58, 1115, 1993) reported that the initial respiration rates for cut apple slices at 2.degree. C. from 12 cultivars varied from 3.5 to 7.6 mL CO.sub.2 /kg/hr. During a 3-day storage period, the respiration rates of these apple slices decreased, but thereafter, only small changes were noted over a 9-day period at 2.degree. C. The acidity of these apple slices decreased progressively over a 12-day storage period at 2.degree. C., presumably due to the respiratory catabolism of the organic acids. Labuza and Breene (J. Food Process. Preserv. 13, 1, 1989) pointed out that the respiration rate of a fresh commodity is related to the level of rapidity of quality deterioration. For high quality retention of cut apple pieces, the respiration rate should be reduced.
When cut apple pieces are exposed to air, the browning of the tissue surfaces becomes apparent within a few hours, and increases over a 2 to 3-day period. The browning is caused by the enzymatic conversion of phenols to quinones, and subsequent non-enzymatic reactions to form brown-coloured melanin polymers. Enzymatic browning of cut apple slices has been controlled industrially by the treatment of the slices with sulfites in solution through the inhibition of polyphenol oxidase activity and reduction of quinones to phenols. Since sulfites can bring about acute allergic reaction in some consumers, some governmental regulatory agencies (e.g. U.S. Food and Drug Administration) have banned the use of sulfites for fresh salad fruits and vegetables. L-ascorbic acid and D-araboascorbic acid (D-erythorbic acid), as reducing agents, can be used as sulfite replacements for inhibiting enzymatic browning of cut apple pieces. Citric acid and calcium salts are used frequently in conjunction with these reducing agents to enhance the inhibitory action. Ascorbic and erythorbic acids interrupt the chemical reaction sequence for browning in cut apples by reducing ortho-quinone compounds to dihydroxyphenolic forms. Citric acid can lower the pH below the optimum level (pH 6.2) of polyphenol oxidase activity. Research reports have indicated that a calcium salt (0.1%) in combination with ascorbic acid (1%) in solution effectively reduced enzymatic browning in apple slices (Ponting, Jackson and Walters, J. Food Sci. 37, 434, 1972). Ascorbic acid or erythorbic acid in the treatment solution for fresh apple slices are usually at levels of up to 1.5% with accompanying supplementary browning inhibitors. Vacuum and pressure infiltration have been used to infuse the browning inhibitors into the tissue of apples. However, such physical treatment causes water-logging. It is apparent that, to impede enzymatic browning of fresh cut apple pieces, treatments to lower pH below the optimum pH of polyphenol oxidase, and to reduce quinones, are minimally essential.
Research results have shown that when apples are cut into slices, the firmness decreases steadily during refrigerated storage. Kim, Smith and Lee (J. Food Sci. 58, 1115, 1993) indicated that, over a 12-day storage period at 2.degree. C., the firmness of apple slices from 12 cultivars decreased by 15.5 to 52.9%. The reduction of slice firmness can be attributed to the breakdown of intercellular and cellular pectic substances. Retention of firmness, crunchiness and crispness of cut apple pieces is dependent on the restriction of ripening and senescence, and the retention of the cellular turgor. Exopolygalacturonase is an enzyme involved in the breakdown of intercellular and cellular pectic substances in ripe apples. Ethylene, the ripening hormone in the apple, triggers the synthesis of this enzyme. The synthesis of ethylene in cut apple pieces can be impeded by CO.sub.2 infusion into the tissue.
During the storage of cut apple pieces, cytoplasmic fluid may exude from the parenchyma tissue onto the cut surfaces. The fluid, derived mostly from the vacuoles of the cells, contains organic acids, sugars and minerals which can be utilized as nutrients for microbial growth. Studies have shown that the vacuolar fluid (juice) of apples is a suitable medium for the growth of yeast, mold lactic acid bacteria and acetic acid bacteria. To preserve the high quality of cut apple pieces for periods up to 6 weeks, microbial growth inhibition through the utilization of impact extrinsic parameters is imperative.
Considerable information has been published on the advantages of modifying the atmosphere around whole fruits to prolong the shelf-life and maintain the fresh quality (Powrie and Skura, in Modified Atmosphere Packaging of Food, Ellis Harwood, 1991). A modified atmosphere is a gas mixture which has a composition different than that of air. Specific levels of oxygen and carbon dioxide surrounding fruits can inhibit the respiration rate, ripening, senescence and microbial growth. Apples in a controlled modified atmosphere of about 2% oxygen and 2% carbon dioxide can be stored for up to 10 months and retain acceptable quality characteristics.
The effectiveness of polymeric plastic films as package material for extending the shelf-life of fresh whole fruits, including apples, with an equilibrium modified atmosphere in the headspace of a sealed package, has been reported. This processing methodology is called modified atmosphere packaging (MAP).
Package systems can be designed to maintain the quality attributes of fresh cut fruit pieces for prolonged storage under modified atmosphere packaging (MAP) conditions. The invention of Powrie, Wu and Skura (U.S. Pat. No. 4,895,729) discloses a MAP method for preparing and preserving fresh, ripe, cut fruit pieces in high gas barrier or gas impermeable container with an input of a specified gas mixture into the headspace prior to sealing. Cut apple pieces can be preserved under the methodology of this patented invention with many of the quality attributes of fresh apple tissue being maintained. However, during the storage of the cut apple pieces, the carbon dioxide content increases to a point where some fizziness (carbonated taste) is perceived. With an extended storage period, the cut apple pieces attain a slight off-flavour. Although the stored cut apple pieces with these quality aberrations were acceptable, the absence of these properties would elevate considerably the sensory score to superior quality.
O'Beirne (Processing and Quality of Foods, Vol. 3, Elsevier Applied Science, 1989) outlined a combination process of modified atmosphere packaging/vacuum packaging, dipping in an ascorbic acid solution (0.5 to 1% ascorbic acid, 1% citric acid and 0.5% calcium chloride) and chilled storage (5.degree. C.) for extending the shelf-life of apple slices. The packaging material was a high gas barrier polymer laminate and the input gases were oxygen-free nitrogen and 50% nitrogen--50% carbon dioxide mixture. With the oxygen-free microatmospheres in the headspace of the containers, the fresh apple slices could be stored for 2 to 3 weeks with acceptable colour and flavour, but became insipid thereafter. With the 1% ascorbic acid, 1% citric acid with 0.1% calcium chloride dip solution, the % free liquid (drip) after 21 days of storage of slices was reported to be between 2.8 and 8.1, depending on the headspace input gas composition.
U.S. Pat. No. 3,754,938, Ponting, Aug. 28, 1973, discloses a process whereby the quality of apple slices is preserved for an extended period of time by the synergistic effect of a treatment solution (pH 7 to 9) consisting of ascorbic acid, calcium chloride and sodium bicarbonate. Treatment with said solution eliminates the use of any sulfiting agent in a process for the preservation of apple slices.
U.S. Pat. No. 4,011,348, Farrier et al., Mar. 8, 1977, discloses a process whereby raw fruits and vegetables are treated with an aqueous solution having a pH between about 4 and 7.5 and containing a selected ascorbate ion concentration in order to maintain desirable colour, texture, odour and flavour characteristics when the fruit and vegetables are stored at aerobic refrigeration conditions for extended periods.
Swedish Patent No. 91-01063, Lundholm, discloses packing fresh, peeled apples with 90 to 95% nitrogen. The process involves coring fruit and placing it in hermetic packing from which air is evacuated prior to filling with nitrogen. An inert gas can be added to the nitrogen, with the total volume of nitrogen alone or nitrogen plus inert gas being up to 90 to 95% of the total gas volume. Once the gas is added, the package is tightly closed. The apples or apple pieces are treated with a food acid, such as ascorbic acid or citric acid, before being placed in the package. They are also blanched. The filled and sealed package is stored at a temperature of preferably 4 to 8.degree. C. until use. The added inert gas or gases amount to approximately 10 vol. %, calculated on the nitrogen content, helium or argon being used. The package itself is opaque.
Japanese Patent No. 4011860, Naganoken Noson Kog, Jan. 16, 1992, discloses a process whereby slices of apple or other fruit are preserved by rapidly immersing the apples or fruit in an aqueous solution of ascorbic acid and packaging in plastic film. The inside of the package is evacuated and N.sub.2 gas is introduced.
Japanese Patent No. 3080044, Dainippon Printing KK, Apr. 4, 1991, discloses a process for preserving cut apple pieces. The washed and peeled apples are cut into several pieces. The pieces are preserved in an aqueous solution of ascorbic acid and/or salt. The products are put into a package with N.sub.2 gas, thus eliminating O.sub.2 gas.
Japanese Patent No. 59006834, Suzuki, Jan. 13, 1984, discloses preserving fruit which tends to brown upon oxygen exposure by dipping it in salt solution and anti-browning solution, packaging it in a gas barrier film after purging the oxygen, and refrigerating. Peeled fruit is rinsed with salt solution, dipped in an anti-browning solution, placed in wrapping material having high gas-barrier properties, sealed in it after purging oxygen, and then stored under refrigeration. Salt concentration of the salt solution is 0.1 to 0.3%. The salt solution is 0.9 to 1.1% salt solution or 0.4 to 0.6% ascorbic acid solution. Peeled fruit with any defective parts removed can be stored for a long time, e.g. a few months, without browning. The fruit may be, for example, apples, pears, persimmons, etc., the flesh of which tends to discolour on contact with oxygen. By rinsing the fruit with the salt solution, brown colour causing substances, e.g. diphenol compounds, are removed from the surface. The anti-browning solution may be a mixture of 0.2 to 0.4% salt solution and 0.4 to 0.6% ascorbic acid solution, or it may be 0.01 to 0.03% solution of sodium hydrogen sulphate, etc. The dipping is performed for 30 seconds and the liquid temperature is kept at 5 to 10.degree. C. The wrapping material may be, for example, nylon film. Oxygen purging may be performed either by vacuuming or N.sub.2 gas substitution.